This invention relates to a method and apparatus for producing an extruded plastic bar, or the like, having at least two longitudinally extending sections around the bar with different surface characteristics.
Plastic sectional bars are used, e.g. in motor vehicles, to a considerable extent as protective or as trim strips. They generally have a uniformly colored surface. Depending on the composition of the material and the temperature of the extruder used in fabricating them, the sectional bars can have a bright to high-mirror finish surface, or a dull or matte surface. There are also sectional bars with surfaces having colored longitudinally extending strips.
Consumers demand sectional bars with surfaces having at least one continuous dull zone extending in the longitudinal direction. The dull zone is particularly desired in sectional bars with colored longitudinally extending strips, where the surface of the colored longitudinally extending strips is to remain dull relative to the remaining surface.
An extruded trim strip of transparent or translucent plastic is available which has both a colored longitudinal strip and a dull surface in this region. This trim strip is produced by co-extrusion using two plastic materials which are adapted exactly to the desired appearance of the surface. Although a sectional bar with different degrees of brightness can be obtained by co-extrusion, this requires a relatively elaborate extrusion tool and an additional extruder, so that the manufacturing costs are quite high.
German Published Application (Offenlegungsschrift ) No. 20 40 713 discloses a plastic molded body that is dulled by coating the molded object with a dull varnish. It seems difficult to apply this to a plastic sectional bar where only a limited width strip dull zone is to be produced. This would at least require an additional operation, which is susceptible to trouble, and there could be a problem regarding the adhesion of the dull varnish to the surface of the sectional bar.
Furthermore, this German application shows the possibility of producing a dull surface in plastic molded objects by dulling the surface of the molding tool used in the manufacture, for example, by etching the molding tool surface. This may bring the desired result in the manufacture of molded plastic objects which are molded by reshaping, but if this were applied to an extrusion tool, it would cause the sectional bars thus produced to have a cracked surface.
German Allowed Application (Auslegeschrift) No. 12 38 196 discloses applying colored strips to the surface of pipes by pressing on colored strips directly after the pipes emerge from the extruder, under slight pressure. This requires storage and separate manufacture of colored strips and involves the use of an apparatus for applying the strips. For these reasons, this considerably increases the cost of the end product.